The present invention pertains generally to high frequency Inductively Coupled Plasma (ICP) torches. More specifically, the present invention pertains to ICP torches which minimize the gas feed that is required to initiate and maintain the atomization or vaporization of molten salt materials. The present invention is particularly, but not exclusively, useful as an ICP torch for a molten salt, such as a multi-component nuclear waste slurry, which includes both a volatile component and a refractory component.
Various types of ICP torches which can produce high temperature gaseous plasmas for such purposes as plasma etching, evaporation of refractory materials, spectroscopy, sintering waste incineration and mitigation are well known. In large part, the wide range of applications for which ICP torches can be used is due to the fact that these torches are generally capable of producing heat loads in excess of 100 MW/m2 on the surface of small particles or droplets injected in the plasma. Another application, among several, which is attracting new attention is the creation of plasmas for the purposes of remediating the refractory components of nuclear waste. Importantly, it is also well known that even the more sturdy refractory materials, such as are found in nuclear waste, will vaporize under heat loads around 100 MW/m2. The challenge in this case is to attain and maintain such heatloads.
In a typical operation, an ICP torch will produce a plasma by ionizing a gaseous substance with a high frequency RF electromagnetic field (i.e. RF. power). For such operations, the gaseous substance in this case is usually referred to as a carrier gas, and the electromagnetic field is typically produced by an induction coil at frequencies in a general range of 0.4-30 MHz. In any case, the result is a high temperature gas flow having temperatures that reach upward to about 10,000-20,000xc2x0 K. It happens, however, that the power density that can be generated in an ICP torch is limited by the heating of the side wall of the plasma torch chamber. Thus, the side wall of the torch chamber should have a high heat conductivity to keep the wall temperature at a sufficiently low operational temperature (e.g. significantly below the range of 10,000-20,000xc2x0 K). At the same time, the side wall should also have a high electrical resistivity to allow for the penetration of an AC electromagnetic field into the plasma chamber.
While the ionization, atomization or vaporization of volatile components can be accomplished using heat loads that are generated at relatively low temperatures (e.g. below 100 MW/m2 and well below the range of 10,000-20,000xc2x0 K), this is not the case for refractory components. In fact, the vaporization of a refractory component will often require heat loads that are in excess of the 100 MW/m2 mentioned above. Consequently, very high temperatures must be accommodated if refractory components are to be vaporized.
One solution to the high temperature problem has been to cool the wall of a plasma torch with a gas vortex that is created by injecting gas tangentially onto the wall. Although such a procedure may be efficacious for the purpose of cooling the chamber wall, it will also contribute to the throughput of the torch. Further, the total throughput will be increased if a carrier gas is used in the ICP torch along with the cooling gas vortex. In some applications, however, these consequences may present a significant disadvantage. For example, in applications where refractory components need to be vaporized, it may be desirable to minimize the amount of gas in the throughput. Specifically, when refractory components are to be vaporized in a plasma torch, it may be necessary that the resultant plasma be transferred to a vacuum chamber for subsequent processing. In such applications, the efficacy of the subsequent processing and the efficiency of vacuum pumps can only be increased by decreasing the amount of gaseous throughput.
In light of the above, it is an object of the present invention to provide an ICP torch and a method for vaporizing a molten salt that contains both a volatile component and a refractory component wherein the volatile component is initially vaporized to create a carrier gas that will heat the refractory component, which will then be vaporized. Another object of the present invention is to provide an ICP torch and a method for vaporizing a molten salt which reduces the gas-to-waste feed ratio to minimize the gas throughput. Yet another object of the present invention is to provide an ICP torch and a method for vaporizing a molten salt which will control the deposit of condensed vapors inside the chamber of the torch. Still another object of the present invention is to provide an ICP torch and a method for vaporizing a molten salt which is relatively simple to manufacture, is easy to use and is comparatively cost effective.
In accordance with the present invention, an inductively coupled plasma torch atomizes a molten salt that contains both a volatile component and a refractory component. More specifically, the molten salt is atomized in the plasma chamber of the plasma torch, in stages. Initially, the plasma torch vaporizes the volatile component of the molten salt to create a carrier gas in the chamber. The torch then uses the heat and pressure that are generated by the carrier gas to promote a subsequent vaporization of the refractory component. The result is a lower gas throughput for the plasma torch. Additionally, the plasma torch is constructed to prevent, or at least minimize, the condensation of molten salt vapors in the chamber that would otherwise adversely affect the operation of the plasma torch.
In the general aspects of its construction, the plasma torch of the present invention includes a cylindrical shaped outer member and a cylindrical shaped inner member that is coaxially positioned inside the outer member. With this configuration, a space is established between the two members. The purpose of this space is actually twofold. First, it is the location for the induction coil which is used to generate r.f. power for the plasma torch. Second, the space also holds a fluid coolant which cools the induction coil, as well as the torch itself. Additionally, the inner member defines an axially elongated chamber.
Depending on the particular mechanism that is used for injecting the molten salt into the chamber, the inner member will be constructed with different configurations. For one embodiment, the inner member will be configured to accommodate a cleaning gas which will enter the chamber and remain near the wall of the inner member. In another embodiment, the inner member is configured to support and carry a film of the molten salt. In either case, the inner wall is constructed to help minimize the gas-to-waste feed ratio and to control deposits on the wall.
In one embodiment of the present invention, the mechanism for injecting the molten salt into the chamber is a nozzle or multiple nozzles. Specifically, the nozzle is designed to spray droplets of the molten salt into the chamber that have diameters which are approximately less than one hundred microns ( less than 100 xcexcm). Additionally, the injection mechanism for this embodiment can include various passageways for directing a cleaning gas, such as sodium vapor or water vapor, over the inner wall. The main purpose of this cleaning gas is to inhibit, or prevent, the condensation of molten salt vapors on the inner wall.
When a nozzle is used to inject a molten salt into the chamber of the plasma torch, the cylindrical inner wall will include a plurality of elongated, preferably copper, segments. Specifically, each segment is aligned substantially parallel to the axis of the chamber, each segment is juxtaposed between two other segments, and each segment is formed with an axially aligned liquid coolant channel. Further, a spacing plate, that is made of an electrically insulating material, is positioned between each pair of juxtaposed segments. Additionally, each segment is provided with a ceramic shield which is mounted on the segment to interface with the chamber. Preferably, the ceramic shield is made of a refractory material which has a low electrical resistivity and a high thermal shock resistance.
For another embodiment of the present invention, the mechanism for injecting a molten salt into the plasma chamber includes a jet which is positioned at one end of the chamber to direct the molten salt tangentially onto the inner wall. In general, the molten salt is injected onto the inner wall with a tangential velocity (vxcex8) that is in a range from about one-half to about two meters per second (vxcex8=0.5 to 2 m/sec). Specifically, this is done to create a film of molten salt which will swirl through the chamber on the inner wall. As the molten salt evaporates from the surface of this film, droplets of the molten salt will react with r.f. power from the induction coil in a manner similar to the nozzle version of the present invention as previously described. Unlike the nozzle version, however, the inner wall for this embodiment will need to be relatively smooth in order to reduce turbulence in the flow of the film. Further, for this embodiment, a diverter is positioned at the opposite end of the chamber from the jet to receive the unevaporated molten salt film from the inner wall. The diverter will then remove unevaporated molten salt from the chamber and redirect the unevaporated molten salt back to the jet for recycling.